Digital micromirror device having a window transparent to ultraviolet (UV) light

ABSTRACT

A UV-transmissable window assembly for a DMD device includes a UV-transmissable glass window provided in a frame. The window and frame are bonded together to preferably effect a hermetic seal therebetween. Optical coatings specific to the intended wavelength of light transmission are applied to the inner and outer surfaces of the glass window to reduce reflection and increase light transmission therethrough. The window assembly, and DMD provided with the same, is adapted for excellent transmission of ultraviolet light, even at the deep ultraviolet portion of the spectrum. The DMD window assembly has application in the medical arts, both surgery and device manufacture, in the production of integrated circuits (IC), and in other optical lithography applications, among other fields.

The U.S. Government has a paid-up non-exclusive license in thisinvention as provided for by Grant No. R44 EY11587 awarded by theNational Eye Institute Division of National Institute of Health.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to optical systems. Particularly, thisinvention relates to micro-electro-mechanical-systems (MEMS) havingoptically reflective and transparent elements, and more particularly, tosuch systems wherein the optically transparent elements are transparentto ultraviolet light wavelengths.

2. State of the Art

The correction of abnormal human vision has progressed rapidly over thepast few years. Although eyeglasses and contact lenses are still thedominant approach for correcting vision, newer techniques involving thereshaping of the cornea, and the replacement or supplement of theinternal human lens, are providing more precise correction. To correctvision via the reshaping of the human cornea, precision surgicalscalpels or lasers are used. Although still used, radial keratotomy(RK), which uses a surgical scalpel, is quickly being replaced byphotorefractive keratectomy (PRK) and laser in-situ keratomileusis(LASIK), which use lasers. The laser refractive surgery field hasexploded over the past few years with many new lasers and algorithms tocorrect human vision. Systems are now using laser wavelengths from thedeep-ultraviolet to the infrared to change the shape of the cornea in acalculated pattern, which makes it possible for the eye to focusproperly.

Artificial intraocular lenses (IOLs), which replace the human lens,usually due to cataract formation or lens damage, also provide very goodvision correction, but, like contact lenses, which can offer “broad”spherical and astigmatic correction, they are limited in the precisionof their corrective power. There are also supplemental refractive-IOLswhich are inserted in the anterior chamber of the eye, the space betweenthe iris and the cornea, while leaving the original lens intact. Lightadjustable IOLs which are able to be altered outside the eye, or withinthe eye after implantation, thus allowing more custom-fit and moreprecise corrective power, are now undergoing research. Such an IOL isdescribed in U.S. Pat. Application 2002/0016629 entitled “Application ofWavefront Sensor to Lenses Capable of Post-Fabrication PowerModification”, which is incorporated by reference herein in itsentirety. Mid-UV to deep-UV lasers are used to alter these IOLs toprovide varying refractive powers.

Finer, more precise measurements of human eye abnormalities have alsobeen improving over the past several years. As these measurements havebeen improving, the industry has searched for ways to generate morecustom corrections to the eye or to the IOL. The Digital MicromirrorDevice™ (DMD™), a micro-electro-mechanical-system (MEMS) semiconductordevice consisting of hundreds of thousands of micromirrors, is an idealdevice to deliver the custom laser beam pattern more precisely. The useof the DMD™ with respect to laser eye surgery is described in detail inco-owned U.S. Pat. No. 5,624,437, entitled “High Resolution, High Speed,Programmable Laser Beam Modulating Apparatus for Microsurgery”, U.S.Pat. No. 6,394,999 entitled “Laser Eye Surgery System Using WavefrontSensor Analysis to Control Digital Micromirror (DMD) Mirror Patterns”,and U.S. Pat. No. 6,413,251 entitled “Method and System for Controllinga Digital Micromirror Device for Laser Refractive Eye Surgery”, all ofwhich are hereby incorporated by reference herein in their entireties.Current, commercially-available DMD devices are designed to delivervisible wavelengths (from 400-nm to 750-nm), however, and cannot be usedto deliver a large range of UV energy because of their protectivewindow, which environmentally guards the micromirrors. UV energy can becategorized by wavelength according to physical definitions: extreme UV(EUV)(10 nm to 100 nm), vacuum UV (VUV)(10 nm to 200 nm, withrecognition that VUV overlaps EUV), far or deep UV (DUV)(200 nm to 300nm), and near UV (NUV)(300 nm to 400 nm). In addition, UV energy can becategorized by wavelength according to photobiologic definitions: UV-C(100 nm to 280 nm) which overlaps far and deep UV, UV-B (280 nm to 315nm) which overlaps far and near UV and is also termed mid-UV, and UV-A(315 nm to 400 nm) which overlaps deep and near UV and which is alsotermed near-UV for photobiologic purposes.

Refractive Surgery: Corneal Reshaping by Laser and the Use of the DMD

Initial systems approved by the FDA for corneal reshaping implement therefractive correction by delivering beam-shaped laser energy based onfirst-order approximations of refraction from a single sphericalsurface. These systems implement a “broadbeam” approach, whereby thelaser beam is shaped by a motorized iris (myopia and hyperopia) andmotorized slit (astigmatism) based on profiles derived throughMunnerlyn's derivation, as discussed in C. R. Munnerlyn, et al.,“Photorefractive keratectomy: a technique for laser refractive surgery,”J. Cataract Refract. Surg. 14, 46–52 (1988). Typical systems using thisapproach on the market are VISX and Summit. More than one million eyeshave been treated in this manner. This system is limited, however, as ittreats a broad area of the cornea all at one time. Eye topography mapsand more recently, wavefront analysis, reveal the eye has many minutevariations across the cornea. See, e.g., J. Liang, et al., “Objectivemeasurement of wave aberrations of the human eye with the use of aHartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A, Vol. 11, No. 7,1949–1957 (1994). Referring to FIGS. 1( a) and 1(b), the broadbeam laserapproach cannot correct these minute variations.

The latest systems being introduced to the market are based on scanninga small laser spot (typically 0.5-mm to 1.0-mm diameter), or acombination of different sized spots, across the cornea in apredetermined pattern to achieve refractive corrections, and are termed“scanning spot systems”. These scanning spot systems differ in that theyare more flexible than the broadbeam approach. Referring to FIG. 2, withthe control of a small spot 10, one can shape different areas of thecornea 12 independently of other areas. These techniques allow for amore general pattern to be applied to the cornea. Typical systems usingthe scanning spot approach are VISX, Autonomous Technologies,LaserSight, and SurgiLight.

However, there are several problems with the scanning spot approach whencompared with the broadbeam approach but the flexibility offered appearsto outweigh these. Some problems include longer refractive surgery time(speed), safety, tracking and surface roughness, which are discussed inmore detail as follows.

With respect to longer refractive surgery times, the scanning spot is aslower approach since a small spot (typically 1-mm diameter) has to bemoved over a wide surface (up to 10-mm for hyperopia). The broadbeamapproach treats the entire cornea for each laser pulse, or treatmentslice. The scanning spot system must deliver several hundred spots pertreatment slice; thus, treatment times can increase.

With respect to safety, the broadbeam laser is inherently safe from atreatment interruption standpoint because the cornea is treatedsymmetrically for each pulse (the iris represents a circle and the slitrepresents a rectangle so that every point on the cornea is treated thesame for each laser pulse). If the procedure is interrupted, you areguaranteed to have some symmetrical spherical or cylindrical correction,which can be continued easily. The scanning spot, with its small spotsize, cannot cover the entire corneal surface with one laser pulse sothat if an interruption occurs, there is no guarantee of a symmetricaletch at that point.

With respect to tracking, in the scanning spot system, the eye needs tobe tracked in order to deliver the spot to the correct point on thecornea as the eye moves. This is not as much of a problem in thebroadbeam system as a broader area is treated with each pulse.

With respect to surface roughness, laser spot overlap tends to createroughness in the resulting etch. While it is necessary to overlap spotsto provide complete coverage for a given ablation zone, regions ofoverlap will be ablated at twice the etch depth per pulse. Thesmoothness of the ablated volume is dependent on the spot overlap and toa lesser extent, the ratio of spot diameter and ablation zone diameter.This problem is not seen in the broadbeam approach.

More recently, eye contour topography is being used to more accuratelyprovide refractive measurements. Current FDA-approved refractive lasersystems do not directly use eye-modeling systems, such as cornealtopographers or wavefront sensors, to create the correct treatmentprofile for the patient's eye. The topographic map is used indirectly bythe surgeon for optimizing the treatment plan (diopter correction andastigmatic axis). There are systems currently going through FDA trialsthat do use corneal topographic surface data to directly guide the lasertreatment algorithm. This is accomplished using some type of scanninglaser spot, as current broadbeam laser refractive surgery systems cannotprovide the laser beam detail required to use the topographic map data.Each eye is individually analyzed as to its contour before ablation isapplied. The idea here is to take into account the varying degrees ofcurvature and height variations across the corneal surface, as opposedto assuming a spherical surface as is currently done in broadbeamsystems. Once these curvatures, or powers, are determined by eyetopography, e.g., by using a system sold by Keratron, Orbtek, orZeiss-Humphrey, they can be considered within the refraction correctionderivation (as described by Munnerlyn) to create a customized ablationpattern for each individual eye. These ablations must be implemented bya scanning spot system, or better yet a DMD approach, as individualareas must be treated differently than other areas. Previouslyincorporated U.S. Pat. Nos. 5,624,437 and 6,413,251 discuss the DMDapproach. Even this approach is limited in that only aberrationsmeasured on the corneal surface are included in the refractivecorrection derivation.

The optimum approach to date uses the recent introduction of wavefrontsensing analysis of the eye. With this new technology, a very powerfulset of tools to correct the eye corneal surface is provided. Wavefrontsensing provides an overall refractive analysis of the entire eyeoptical system, e.g., taking into account the cornea, the lens, thevitreous and the retina. The result of a wavefront sensor analysisyields a waveform model that represents a nearly perfect refractionmeasurement. This provides a superior analysis of the eye versus thecurrent topography systems that only analyze the cornea. Wavefront datamay be used to drive a scanning spot system, but it still encompassesthe problems discussed before. However, it is directly compatible withthe DMD approach due to the digital nature of the wavefront sensoranalysis. This wavefront sensor-DMD approach is discussed in detail inpreviously incorporated U.S. Pat. No. 6,394,999.

Refractive Surgery: Light Activated IOLs and the Use of the DMD

Modern cataract surgery, in which the cataract is actually extractedfrom the eye, was introduced by Jacques Daviel in Paris in 1748. SamuelSharp of London introduced the concept of intracapsular cataract surgeryin 1753 by using pressure with his thumb to remove the entire lensintact through an incision. Small suction cups (erysiphakes) wereintroduced for this purpose in 1902 as well as various capsular forcepsto grasp the lens for removal. Henry Willard Williams of Boston firstdescribed the use of sutures for cataract surgery in 1867. In the 1840sgeneral anesthesia was introduced for surgical procedures, however itwas not until 1884 that anesthesia in the form of eye drops (cocaine)was developed, obviating the hazards of general anesthesia and itspostoperative complications. After Harold Ridley introduced theintraocular lens in England in the 1940s, efficient and comfortablevisual rehabilitation became possible following cataract surgery.

The intraocular lens, or IOL, is a permanent plastic lens implantedinside the eye to replace the crystalline lens. In 1957 Barraquer ofSpain used alpha-chymotrypsin to enzymatically dissolve the zonules forremoval of the lens. Cryosurgery was introduced by Krawicz of Poland in1961 to remove the lens with a tiny probe that could attach by freezinga small area on the surface of the cataract. In the late 1960s CharlesKelman of New York developed a technique for emulsifying the lenscontents using ultrasonic vibrations and aspirating the emulsifiedcataract. In recent decades, there has been a rapid evolution ofdesigns, materials, and implantation techniques for intraocular lenses,making them a safe and practical way to restore normal vision at thetime of surgery.

More recently, designs have been implemented to provide accommodationwith an IOL. Such attempts include diffractive multifocal lenses,flexible (fluid-filled) lenses, multi-element designs and hinged optics.These IOLs still only offer “broad” fixed power correction (bothspherical and astigmatic), although they do offer some accommodativepower.

Even more recently, light-activated IOLs have been proposed. Theseoptical elements have a refraction modulating composition dispersed in apolymer matrix. The refractive modulating composition is capable ofstimulus-induced polymerization, e.g., a UV light stimulus (longerwavelength UV: 325-nm to 340-nm range). In this way, an opticalmeasurement (e.g., topography, wavefront, etc.) of the eye can be madefollowed by inducing an amount of polymerization of the refractivemodulating composition, wherein the amount of polymerization isdetermined by the optical measurement. Thus, the varying degrees ofcurvature and height variations across the corneal surface can be takeninto account, as opposed to assuming a simple spherical surface.Currently, collimated light from a Xe:Hg arc lamp (340-nm through a 1-mmphotomask), or collimated light from a He:Cd laser (325-nm, 1-mm beamdiameter), is used to activate and stabilize (or “lock in”) therefractive modulating composition. To polymerize the entire IOL, the1-mm photomask, or 1-mm laser beam, must be moved or scanned across theIOL, as described in U.S. Pat. Application 2002/0016629 A1 entitled“Application of Wavefront Sensor to Lenses Capable of Post-FabricationPower Modification”, which is incorporated by reference herein in itsentirety. Scanning the small masks or small diameter laser beams acrossthe IOL present the same problems as described in the scanning spotapproach to corneal tissue reshaping discussed above. Therefore, whencoupled to a proper collimated broadbeam arc lamp, or a broadbeam laser,this technique is directly compatible with the DMD approach due to thedigital nature of the topographic or wavefront sensor analysis, andsince it can provide larger, custom laser beam patterns more precisely.

The DMD Problem for Ultraviolet Wavelengths

From the above discussion, it is apparent that the DMD is ideally suitedto provide the necessary laser delivery control, in both the cornealreshaping application and the IOL activation application, to obtain thefiner resolution and custom laser beam patterns required by the moreadvanced measurement techniques. To date, however, there has not been acommercially-available DMD to provide delivery of the excimer wavelengthused in laser refractive surgery (193-nm) or the wavelengths used in thelight-activated IOL approach (325-nm to 340-nm). The shortest optimizedwavelength able to be delivered by a manufactured, but stillexperimental DMD is 365-nm, far above the 193-nm necessary to etch thecornea in the laser refractive surgery area.

Referring to FIG. 3, it is noted that at 325-nm, a wavelength used inthe IOL application, only about 83% of the light is transmitted throughthe UV-coated DMD window in a single pass. Moreover, with the DMDdevice, the light that strikes the mirrors must make two passes throughthe window; that is, the light must travel through the window first,reflect from the mirrors and travel through the window again to exit thedevice. This means that only about 69% (0.83×0.83) of the original325-nm light returns from the DMD, assuming 100% reflection from the DMDmirrors, or a loss of 31% due to the window alone. At 340-nm, only about91% of the light is transmitted in a single pass. Thus, for 340-nm, onlyabout 83% (0.91×0.91) of the original 340-nm light returns from the DMD,assuming 100% reflection from the. DMD mirrors, or a loss of 17% due tothe window alone. The result of this loss is that the laser source mustoperate at higher energy output levels, which also increases damage tothe beam shaping and delivery optics. Furthermore, there are otherlosses associated with the optics that are required to shape,homogenize, and deliver the laser beam (typically 45% to 60% loss).Additional losses result from damage to the optical coatings as a resultof use.

To further illustrate the window losses, consider an example from U.S.Pat. Application 2002/0016629 A1, where a 1-mm diameter, 325-nm He-Cdlaser beam with an energy density of 257 mJ/cm² is required to inducethe refractive modulating composition polymerization. To cover theentire 6.35-mm IOL with the laser beam at one time, as would benecessary using a DMD to activate the refractive modulating composition,this would require 81 mJ of energy at the IOL. With the above windowlosses of 31% at 325-nm, coupled to the typical losses of 50% for thebeam shaping and delivery optics, this would require over 500-mJ fromthe laser, while operating from two minutes to ten minutes. Thus, itwould be advantageous to have a deep UV window as lossless as possibleto keep the laser energy requirement down.

As another example, consider the energy density required to etch cornealtissue. Although energy densities vary from system to system, a typicalvalue is 160 mJ/cm². A current commercially-manufactured, butexperimental, DMD will not work for the laser refractive surgerywavelengths of between 190-nm to 250-nm (typically 193-nm) because asseen from FIG. 3, the UV-coated window covering the mirrors of the DMDhas a 0% optical transmission below 250-nm. Thus, the only way toimplement the DMD for this application is to use a window designed forthese deep-UV wavelengths. For a typical 6-mm spot used in laserrefractive surgery, an energy density of 160 mJ/cm² requires 45-mJ. Fora 10-mm spot, used to correct hyperopia, 125-mJ is required. Typicaltreatment times for broadband lasers range from a few seconds to oneminute. Thus, the deep UV window needs to be as lossless as possible tokeep the laser energy requirement down.

Another significant problem that exists with current DMD window designsis the reflections from the window surfaces, particularly on the DMDmirror side of the window. If the window and its coating are notoptimized for the wavelength used, reflections can bounce back and forthfrom the inside of the window to the mirrors giving rise to “ghostimages” that can cause interference problems resulting in incorrectpatterns being delivered.

Current DMD windows assemblies are constructed of a Kovar® (ASTM-F-15)metal alloy frame and a borosilicate glass window. This combination is acommon glass-ceramic sealing systems for protecting semiconductors(e.g., EPROMS) from a local environment. Kovar® is a low-expansion alloywhose chemical composition is controlled within narrow limits to assureprecise uniform thermal expansion properties.

The most common borosilicate glass used in the current DMD applicationis Corning 7056. Corning 7056 glass works well for the visible lightspectrum DMD application because it passes visible light well and itscoefficient of thermal expansion (CTE) is very close to Kovar® (Corning7056: 5.15×10⁻⁶/° C. versus Kovar®: 5.2×10⁻⁶/° C.). This allows aglass-to-metal hermetic seal when both are heated to nearly 1000 degreesCelsius. The traditional hermeticity definition is based on the HeliumFine Leak Test (mil-std 803 or JEDIC-JESD22-A109-A) where the value mustbe 5×10⁻⁸ atm-cc/s helium or better. The hermetic seal is formed byheating both the glass and metal until a wetting of the metal by theglass occurs, followed by the development of a chemical bond or somemechanical interlocking, thus maintaining the seal. The basetransmission spectrum of Corning 7056 is shown in FIG. 4. By applyingappropriate anti-reflection (AR) coatings to the glass, the transmissionspectrums of FIG. 3 are achieved. Note the optical transmission can beshifted lower to handle the near-UV wavelengths, but this degrades partof the visible spectrum. Finally, note again that this glass will notpass deep-UV to mid-UV wavelengths very effectively. Instead a differentmaterial, such as fused silica, is required.

Fused silica is one of the most common materials used in the deep-UV tomid-UV applications. Fused silica is a polycrystalline, isotropicmaterial with no crystal orientation. Its physical, thermal, dielectricand optical properties are uniform in all directions of measurement.There are special grades of fused silica, termed excimer grade fusedsilica, made especially for the above applications. Unfortunately, theCTE of fused silica (0.55×10⁻⁶/° C.) is not very close to the CTE ofKovar® (5.2×10⁻⁶/° C.) (differing by substantially an order ofmagnitude), and thus during the manufacturing process, and in thepost-manufacturing environment, as temperatures vary, the hermetic sealbetween the two is not maintained. This allows the outside environmentinto the hermetically-sealed DMD semiconductor space and this becomesdetrimental to the micromirrors behind the window causing them not tofunction properly. One of the most common problems is that the mirrorsstick and do not respond to commands.

There are three additional problems with respect to current DMD mirrordesign. First, the current commercially-available DMD device uses barealuminum mirrors to reflect the incoming light. Uncoated, or bare,aluminum provides about an 85% absolute reflectance from 200-nm to2000-nm. This reflectance increases as wavelengths move into the visiblearea (about 90% averaged over 400-nm to 750-nm), as in the mainapplication of the currently-manufactured DMD devices, and decreases aswavelengths move below 200-nm (e.g., about 84–86% for 193-nm and about65–70% for 157-nm). Therefore, the uncoated aluminum DMD mirrors provideless than 85% reflectance, or greater than a 15% loss, for certain UVapplications.

Second, the UV energy that strikes the mirrors also gradually erodes themirrors, which damages or deforms them. This negatively affects thelaser energy pattern that is delivered to the target.

Third, the incoming UV energy will travel between the mirrors andimpinges on the underlying semiconductor control structure behind themirrors. This can lead to degradation of the DMD device.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a window assemblyfor a DMD that is substantially transparent to ultraviolet light.

It is another object of the invention to provide a window assembly for aDMD that is substantially transparent to wavelengths of light in thenear, deep, and vacuum ultraviolet portions of the spectrum.

It is a further object of the invention to provide a window assembly fora DMD that has minimal reflectance.

It is also an object of the invention to provide a window assembly for aDMD that includes a window hermetically sealed to a frame.

It is an additional object of the invention to provide a DMD having awindow suitable for applications which use light at ultravioletwavelengths, and particularly in the near, deep and vacuum ultravioletwavelength portions of the spectrum.

It is yet another object of the invention to provide a DMD for use withultraviolet wavelengths, wherein the DMD window seals are not subject tooutgassing problems and thus will not negatively affect the DMD mirrorsor the control electronics.

It is yet a further object of the invention to provide a DMD windowconstruction suitable for use with ultraviolet light and which may besafely cleaned with typical optical cleaning solutions, such as acetoneand methanol.

It is yet an additional object of the invention to increase UV lighttransmission and reflection through the entire DMD system.

In accord with these objects, which will be discussed in detail below, anear-, deep-, and vacuum-UV wavelength window assembly for a DMD device,and a DMD device incorporating the same, is provided. The windowassembly includes a fused silica glass window provided in a hightemperature metal alloy frame. In a preferred embodiment, the fusedsilica glass window is an Argon fluoride grade fused silica, and theframe is made from a nickel-cobalt-iron alloy such as Kovar®. Alead/silver alloy bonding material interface is provided at the junctureof the window of the frame and provides a hermetic seal between thealloy and the glass. Optical coatings specific to the intendedwavelength of light transmission are applied to the inner and outersurfaces of the glass window to reduce reflection and increase lighttransmission therethrough.

The resultant window assembly, and DMD provided with the same, isadapted for excellent transmission of ultraviolet light throughoutvacuum-UV, deep-UV and near-UV wavelengths. A DMD with a window adaptedfor such ultraviolet light transmission has application in the medicalarts, in both surgery and the manufacture of medical devices such asintraocular lenses, contact lenses or eyeglasses, or to selectivelyalter the bio-response of a surface; in the production of integratedcircuits (IC) and in other optical lithography applications (such aspolymer arrays); the custom manufacture of industrial lenses;micromachining (e.g., microhole drilling, selective thin-film removal,and milling); and precise surface roughening of material, among otherfields.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Prior art FIG. 1( a) illustrates minute variations on a corneal surface;

Prior art FIG. 1( b) illustrates a broadbeam laser ablation of thecorneal surface of FIG. 1( a), showing how minute variations of thesurface are maintained after broadbeam laser ablation;

Prior art FIG. 2 illustrates a scanning spot approach showing theflexibility of firing many small spots;

Prior art FIG. 3 shows a graph of current, commercially-available DMDwindow transmission spectrums;

Prior art FIG. 4 shows a transmission spectrum of borosilicate glassused in current DMD application;

FIG. 5( a) is a bottom perspective view of a window assembly accordingto the invention;

FIG. 5( b) is an exploded view of a window and frame of the assembly ofFIG. 5( a);

FIG. 5( c) is a bottom perspective view of the top of the window andframe of the assembly of FIG. 5( a);

FIG. 5( d) is a top perspective view of the bottom of the window andframe of the assembly of FIG. 5( a);

FIG. 5( e) is a plan view of a DMD provided with the window assembly ofFIG. 5( a);

FIG. 6 shows external transmission characteristics of HPFS, Corning7980, ArF-grade fused silica window material;

FIGS. 7( a) and 7(b) illustrate the angles-of-incidence of lightentering and exiting a DMD provided with a UV-light transparent windowaccording to the invention, where FIG. 7( a) is for a 16-micron mirrorconfiguration, and FIG. 7( b) is for a 13.7-micron mirror configuration;

FIG. 8 shows a reflectance curve for a multiple-layer dielectric stackV-coating, optimized for a 0° or normal angle of incidence, showing lessthan 0.5% reflection per window surface at 193-nm;

FIG. 9( a) shows a basic multi-layer AR, V-coating, optimized for asingle wavelength;

FIG. 9( b) shows destructive interference due to layers of the V-coatingof FIG. 9( a);

FIG. 10 is a schematic side view of a first alternate DMD assemblyaccording to the invention;

FIG. 11( a) is a schematic side view of a second alternate DMD assemblyaccording to the invention;

FIG. 11( b) is a schematic exploded view of the second alternate DMDassembly of FIG. 11( a);

FIG. 12( a) is a schematic side view of a third alternate DMD assemblyaccording to the invention;

FIG. 12( b) is a schematic plan view of the third alternate embodimentof FIG. 12( a);

FIG. 12( c) is a schematic side view of a variation on the DMD assemblyof FIG. 12( a);

FIG. 13( a) is a schematic side view of a fourth alternate DMD assemblyaccording to the invention;

FIG. 13( b) is a schematic exploded view of the fourth alternate DMDassembly of FIG. 13( a);

FIG. 14 is a schematic side view of a fifth alternate DMD assemblyaccording to the invention;

FIG. 15 is a schematic side view of a sixth alternate DMD assemblyaccording to the invention;

FIG. 16 is a schematic side view of a seventh alternate DMD assemblyaccording to the invention;

FIG. 17 is a schematic side view of an eighth alternate DMD assemblyaccording to the invention;

FIG. 18 is a schematic side view of an ninth alternate DMD assemblyaccording to the invention; and

FIG. 19 illustrates high reflectance (HR) coating of the DMD mirrors andunderlying structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIGS. 5( a)–5(e), according to a preferred embodiment ofthe invention, a UV-transparent DMD window assembly 20 for a DMD™ device50 includes a fused silica glass window 22 provided in a hightemperature metal alloy frame 24. A bonding material 26 is provided atthe juncture of the window 22 and the frame 24.

In the preferred embodiment, the fused silica glass window 22 ispreferably argon fluoride (ArF) grade available from Corning (HPFS® ArFgrade fused silica, Corning Code 7980). The minimum surface quality ispreferably specified with a surface figure of λ/10 at 633-nm, a surfacequality of 10/5 S/D (scratch/dig) and a parallelism of less than 3arc-minutes. The base (uncoated or bare) external transmission spectrumof the ArF grade fused silica window is shown in FIG. 6. From FIG. 6, itis noted that ArF grade fused silica has at least 90% transmissibilityof UV-wavelengths at 185-nm or above. Alternatively, other types ofexcimer grade fused silica are readily available from such companies asCVI Laser, Coherent Auburn Group, and Acton Research/Roper Scientific,and can be substituted.

The high temperature alloy for the frame 24 is preferably anickel-cobalt-iron alloy such as Kovar® (ASTM F15), which has acomposition of 29% nickel, 17% cobalt, 0.30% manganese, 0.20% silicon,0.02% carbon, and a balance of iron.

The preferred bonding material 26 is a lead/silver alloy (approximately97.5% lead and 2.5% silver). The lead/silver alloy is preferred becausethe rectangular shape (i.e., sharp corners) of the preferred windowframe 24 creates stresses due to the difference in CTEs that areaccommodated by the relatively ductile lead/silver alloy and, thus,helps maintain physical hold on the glass. Other lead-based alloys, suchas lead/copper, lead/nickel, lead/titanium, and lead/tin can also beused. In addition, alloys of tin, including tin/silver, tin/antimony,tin/silver/copper, tin/copper, tin/silver/copper/antimony,tin/copper/antimony/silver, tin/silver/bismuth, and tin/bismuth can alsobe used, but generally have higher melting points (which begins toamplify the thermal expansion differences of the materials involved),are less ductile than lead-based alloys, and exhibit poor wetting of thefused silica window. Indium-bearing solders compare favorably well tolead-bearing solders in terms of ductility, melting temperature andstrength and other physical properties, but are relatively expensive.

Optical coatings, discussed in detail below, are applied to the innerand outer surfaces 44, 42, respectively, of the glass window 22. Suchoptical coating is preferably applied by an optical coating specialtycompany, such as Acton Research/Roper Scientific of Acton, MA; ClevelandCrystal of Highland Heights, Ohio; or CVI Laser of Albuquerque, N.Mex.,among others.

The window assembly 20 is assembled as follows. First, the hightemperature metal alloy window frame 24 is constructed in a shape andsize to correspond to the DMD. One reason Kovar® is the preferredmaterial is because it can be readily machined to the exact dimensionsrequired and also because it is the typical material for suchapplications. Such dimensions can be obtained from a DMD manufacturer,such as Texas Instruments. In another approach, the Kovar® frame can beobtained directly from the manufacturer. In either case, the basic frameis modified by chamfering around the bottom interior edges (at 32).

The ArF grade of fused silica window 22 is constructed in a sizedesigned to fit in the frame 24 while allowing channel space for theintermediary lead/silver alloy brazing material, e.g., the windowmaterial is chamfered along its edge 34. When the window 22 is placedwithin the aperture 36 of the frame 24, the chamfered edges 32, 34 ofthe frame 24 and window 22 define a preferably symmetrical channel 38which allows for the lead/silver alloy bonding material 26. In thepreferred embodiment, the fused silica material 22 is arranged in arectangular configuration (preferably with rounded corners to reducestress), although a square (with preferably rounded corners) or circulararrangement is possible, provided that the frame 24 is machined for thatshape and the resulting window aperture 36 allows light to pass to allthe DMD mirrors of the mirror array 52 of the DMD 50 (FIG. 5( e)).

The window 22 is next bonded with the frame 24. To that effect, thechamfered edges 34 of the window 22 are preferably roughened and thenpainted with a paint containing a titanium constituent. The lead/silverbrazing alloy 26 is then provided in the channel 38, and the window 22,frame 24, and brazing alloy 26 are heated together to the eutectictemperature of the brazing alloy 26 (approximately 305° C. for thepreferred lead/silver alloy). During heating, a bond is formed betweeneach of (1) the paint and the glass, (2) the paint and the lead/silveralloy and (3) the lead/silver alloy and the frame, resulting in ahermetic seal of the window to the frame. The heating may occur duringor after the alloy 26 is provided into channel 38. The seal is thenpreferably tested for leaks using a helium leak test down to 2×10⁻¹⁰atm-cc/sec, at a pressure differential of one standard atmosphere and at0° C.

The light transmission of window 22 is then optimized for the particularwavelength, or wavelengths, required (which will depend upon theapplication for which the DMD will be used) by the application ofcoatings, discussed below, to the outside 42 and inside 44 of the window22. Before applying the coatings, a physical mask is preferably placedin front of the sealed window assembly to ensure the coating materialdoes not extend beyond the window onto the frame or the alloy bond.There are several known methods for applying the coatings. In apreferred embodiment, one wavelength is selected and an anti-reflectioncoating is designed and applied to both surfaces of the window. Forexample, in the 193-nm application, the uncoated ArF-grade fused silicawindow has only ˜4.7% reflection loss per surface (with there beingfront and back surfaces for each window), or about 17.5% loss in adouble pass application (like the DMD application). In order to increasethe window transmission, an anti-reflection (AR) coating is added thru adeposition process after the bonding process. For example, a simplemagnesium fluoride (MgF₂) coating placed onto the outside 42 and inside44 of the window, optimized for 193-nm and the correct angle ofincidence, reduces the single surface reflection to ˜1.7% at 193-nm (oran overall transmission loss of 6.63% for the double pass application)yet is still very durable to typical cleaning methods and chemicals.

The coating thickness is selected generally by starting with a λ/4thickness (193×10⁻⁹/4=48.25×10⁻⁹ meters) for a beam striking normal tothe surface. For a non-normal incident beam application, the effectivedifference in optical path length within the coating thickness for thenon-normal incident beam must be considered. In the present application,the coating can be optimized for the exit angle (0° or normal to thewindow) as opposed to the entry angle (20° for a 16 micron DMD mirror(FIG. 7( a)) or 24° for a 13.7 micron DMD mirror (FIG. 7( b)). Thisminimizes reflections of the beam on exit of the DMD-window where itcould be detrimental to have reflections bouncing between the mirrorsand the inside window surface. However, ideally the AR coating should beoperable over an exit angle of incidence in the range of 0° to 10° (16micron mirror) or 0° to 12° (13.7 micron mirror), with such rangeimproving antireflectance on the incoming beam while adequately reducingthe inside reflectance.

A “cold” deposition process, such as sputtering, is preferred forapplication of the AR coatings. In the sputtering technique, positiveenergetic particles formed from a plasma bombard the target coatingmaterial and through momentum transfer sputter atoms of the target as avapor that is then bonded to the substrate. Sputtering can produceuniform coatings over large areas, and uses the deposition material moreefficiently than evaporation techniques. Other deposition techniques canbe used, as long as the deposition process temperature does not reachthe brazing alloy eutectic temperature. That is, deposition techniquesthat use high temperatures are less preferable, as such techniques mayresult in temperatures above the brazing alloy melting point and thusmay compromise the hermetic seal.

In order to gain even more transmission, the preferred embodiment uses amultiple-layer dielectric stack coating, optimized for a 0° to 12° angleof incidence, although optimization with respect to another degree ofangle-of-incidence, such as the 0° to 10° angle shown in FIG. 7( a), canbe used. Such a stack coating is available from Acton Research/RoperScientific of Acton, Mass., and offers better than 0.5% reflection(99.5% or better transmission) per window surface at 193-nm with a 0°angle of incidence (or an overall loss of 1.98% in a double passapplication), as shown in FIG. 8. These narrowband antireflection (AR)coatings, often called “V” coatings, are proprietary to the coatingmanufacturer, e.g., materials used, number of layers, design of layerthicknesses, coating material deposition techniques, computeroptimization algorithm, etc. In general, however, V-coatings aremulti-layer AR coatings that reduce the reflectance of a component tonear-zero for one specific wavelength. V-coatings are extremelysensitive to both wavelength and angle-of-incidence. The basicmulti-layer AR V-coating, optimized for a single wavelength, is termed aquarter/quarter coating. In its simplest form, a quarter/quarter coatingconsists of two layers, both of which have an optical thickness of aquarter wave length at the wavelength of interest. The outer layer ismade of a low-refractive-index material, and the inner layer is made ofa high-refractive-index material (as compared to the substrate, such asArF-grade fused silica). As shown in FIG. 9( a), wavefront B andwavefront C (second and third reflections, respectively) are bothexactly 180-degrees out of phase with wavefront A (the firstreflection). The performance of the coating is calculated in terms ofthe relative amplitudes and phases, which are then summed to give theoverall, net amplitude of the reflected beam. In a perfect design, thisresult would be zero-reflectance, as indicated by the “resultant wave”in FIG. 9( b). The general example in FIGS. 9( a) and 9(b) describes acrown glass substrate with an index of refraction, n₃, of 1.52; a MgF₂first layer with an index of refraction, n₁, of 1.38, and a second layerwith an index of refraction, n₂, of 1.70 (such as beryllium oxide ormagnesium oxide). The formula for exact zero reflectance for such a twolayer coating at normal incidence is:

$\frac{\left( n_{1} \right)^{2} \cdot n_{3}}{\left( n_{2} \right)^{2}} = n_{0}$where n_(o) is the index of refraction of air (approximated as 1.0). Inthe preferred embodiment, ArF-grade fused silica is used for the 193-nmapplication. It has an index of refraction, n₃, of 1.56. Using a firstMgF₂ layer to provide a durable protection layer, a second coating layerpossessing an index of refraction of 1.72 is required in order toachieve zero reflection. Although this approach allows some freedom inthe choice of coating materials and can give very low reflectance, thequarter/quarter coating can sometimes be too restrictive in the design,e.g., if a suitable coating cannot be found with the correct index ofrefraction, or the angle of incidence of the incoming light is notnormal to the surface of the glass. For these cases, an alternativemethod can be used.

In the alternate method of generating a multi-layer AR coating, thelayers have different thicknesses. This allows one to adjust the layersto suit the refractive index of available materials, instead of viceversa (as above). For a given combination of materials, there areusually two-combinations of layer thicknesses that will give near-zeroreflectance at the design wavelength. These two combinations are ofdifferent overall thickness. This method also aids in the design ofcoatings when the angle of incidence of the incoming light is not normalrelative to the surface. However, two main issues lead to a complicateddependence of reflectance, and thus transmission, on the angle ofincidence. First, the path difference of the front and rear surfacereflection from any layer is a function of angle. As the angle ofincidence increases from 0° relative to a normal relative to thesurface, the optical path difference is decreased. The change in pathdifference results in a change of phase difference between twointerfering reflections. Second, the reflectance of any opticalinterface varies according to the angle of incidence, so when combined,the phase difference between two pertinent reflections changes togetherwith their relative amplitude. Thus, multi-layer coating design atarbitrary angles of incidence is complex. Appropriate computer modelingand optimization algorithms can be performed by optical coatingcompanies, such as CVI Laser, Coherent Auburn Group, Melles-Griot, andActon Research/Roper Scientific, to provide coating materials with theappropriate properties, adherence, stresses, durability, etc. Inaddition, the coatings can be optimized for a dual angle of incidence,such as for both (1) a 20° or 24° entry angle and (2) a 0° exit angle.

In another embodiment, the AR coating may be designed to pass dualwavelengths using the same window substrate material. For example, asimple 193-nm optimized MgF₂ coating placed onto one side of theArF-grade fused silica window can reduce the single-surface reflectionto ˜1.7% at 193-nm and ˜2.75% at 365-nm, with reflection of thewavelengths therebetween likewise substantially reduced as well.Multi-layer, dual-wavelength VV coatings can be applied to achieve evenlower reflectance (more transmission) at the desired wavelengths. Thisgenerally requires additional coatings (stacks) and must be optimizedwith a computer algorithm by a company expert in such coatings, such asmentioned above.

Finally, the optically-coated, hermetically-sealed window unit isinstalled onto the DMD base in the normal manufacturing processing ofthe DMD semiconductor package. For example, the DMD chips are separatedfrom the wafer, plasma cleaned, relubricated and hermetically sealedwith the present invention. This is preferably performed using aparallel resistance seam welding process, although other types ofsemiconductor welding processes may be used. See, Hornbeck, “DigitalLight Processing™ for High Brightness, High resolution Applications,”Electronic Imaging EI '97, Projection Displays III, San Jose, Calif.(Feb. 10–12, 1997). A base 50 includes a port (not shown) at which cable54 can be coupled to permit data transmission from a processor to themirror array 52 to effect configuration of the mirrors in the array intodesired patterns.

According to another embodiment of the invention, the window frame isconstructed from a material significantly different from Kovar®. Theframe material may be either silica fiber orcopper/continuous-carbon-fiber alloy, which have a CTE relatively closeto the UV-transmissable window material and can be machined to definethe necessary shape for the DMD package. The window material can befused silica, as previously described, or another suitableUV-transmissable material, as discussed below. The wet-bonded window andframe assembly are preferably joined using a technique such as activesolder alloy processing (e.g., S-BOND™ available from MaterialsResources International in Landsdale, Pa.) or with low vapor epoxy tothe base DMD body.

Referring to FIG. 10, according to yet another embodiment, the windowframe can be eliminated entirely, with a window 122 of appropriate sizeand shape bonded either directly to the DMD base 150 or to a rectangularKovar® seal ring 152 (a shoulder located on the DMD base and used in theprior art as a spacer between the DMD base and the Kovar® window frame,and as the fusing union therebetween). For example, a low vapor pressureepoxy 154 can be used to effect a seal between the window and theceramic base of the DMD. One such suitable epoxy 154 is Tra-Bond 2116available from Tra-Con of Bedford, Mass. Other suitable epoxies areavailable from Masterbond of Hackensack, N.J. While the epoxy bond maynot be considered a true hermetic seal, as the bond will rarely exceed a2×10⁻⁸ atm-cc/sec helium leak test, the bond does meet the mil std 803spec of less than 5×10⁻⁸ atm-cc/sec, which is nonetheless a very goodseal which is relatively easily accomplished. To effect the seal, it ispreferable to mix and apply the epoxy 154, position the window 122, anduniformly press the window 122 against the DMD base 150 or seal ring 152until the epoxy 154 is cured.

Referring to FIGS. 11( a) and 11(b), if, based on the chosen materials,a single epoxy will not bond to both the window 222 and the DMD base 250or seal ring 252, an intermediate material 260, such as a plastic, maybe provided between the window 222 and the base 250 or the seal ring252, with a first epoxy 254 providing a bond between the window 222 andthe intermediate material 260, and a second epoxy 256 providing a bondbetween the intermediate material 260 and the base 250 or seal ring 252.Optionally, as discussed further below, an elastomeric o-ring 262 can beprovided between the window 222 and the base 250 or seal ring 252 toeffect a true hermetic seal (FIG. 11( b)).

Turning now to FIGS. 12( a) and 12(b), according to yet a furtherembodiment, the window 322 and DMD base 350 (and thus the seal ring 352)are clamped about an elastomeric seal, e.g., a vacuum grade o-ring 362,which provides a hermetic seal therebetween. Suitable elastomers includerubber, butyl, ethylene propylene, and fluorocarbon materials, such asViton® available from DuPont Dow Elastomers. The o-ring 362 can beplaced in various locations, such as on, inside, or outside the Kovar®seal ring 352. Due to the possible lack of large clamping force on theo-ring 362 (discussed further below) and/or because the existing Kovar®seal ring 352 on which the o-ring 362 may lie may not be perfectly flat,the o-ring seal may require a modification to aid in sealing.

Referring to FIG. 12( c), to that end, an intermediate material 364 thatis extremely flat may be used over the current Kovar® seal ring 352between the ceramic body 350 and the window 322, with the intermediatematerial 364 being bonded to the Kovar® seal ring 352 with low vaporepoxy 354. This approach provides a true hermetic seal. The clampingforce to press the window and DMD base against the o-ring, can beeffected in several ways, three of which are discussed, as follows.

First, referring still to FIGS. 12( a) and 12(b), a clamp 370 sits onthe window 322 and includes a port 372 in the center thereof that allowslight to pass in and out of the window 322. The DMD base 350 is coupledto a heat sink 374. The clamp 3703 includes preferably two arms 376, 378that extend in a C-shape from the front of the clamp to around back ofthe DMD base and preferably to the heat sink 374. Alternatively, thearms 376, 378 can extend to only the back of the base 350 of the DMD.Screws 380 are used to uniformly apply pressure across the window 322where it contacts the o-ring 362. There are a number of differentmethods for carrying this out.

Still referring to FIGS. 12( a) and 12(b), according to a firstassembly, the o-ring 362 is applied first, and the window 322 isproperly positioned on the o-ring. Then, the clamp 370 is applied,holding the window 322 against the o-ring 362. Finally, the screws 380are threaded relative to the clamp arms 376, 378 to pull the window 322uniformly against the o-ring 362 to form a hermetic seal.

Referring to FIGS. 13( a) and 13(b), according to a second assembly, theo-ring 462 is positioned on the ceramic DMD base 450 or on the seal ring452. Next, a preferably low vapor epoxy 454 is provided on at least oneof the window 422 and either the DMD base 450 or the Kovar® seal ring452. Then, the window 422 is properly positioned on the o-ring 462.Finally, the window 422 is uniformly pressed against the o-ring 462 andepoxy 454 and held until the epoxy cures.

Referring to FIG. 14, according to a third assembly, a metal rectangularbase ring 582 is bonded to the DMD body 550 peripherally of the sealring (not shown). The base ring 582 has a substantially flat and levelupper surface with a surface area greater than the topmost surface areaof the seal ring. The base ring 582 also has several tapped holes withinit to accept screws. The base ring 582 is bonded with an epoxy 554either directly onto the seal ring, directly to the ceramic body 550, orto both for added sealing capacity. The metal base ring 582 guarantees asmooth, level surface so that the o-ring 562 has a very good surface towhich to adhere and to seal. There are three approaches to attaching thewindow 522 to the base ring 582.

In a first approach, a top metal frame 584 can be used around the window522. The frame 584 is preferably not bonded, but simply includes anotched opening (not shown) which defines a shelf around the frame thatholds the window 522. As the seal is made between the window 522 and theo-ring 562 seated on the base ring 582, this top frame 584 around thewindow 522 does not need to seal. The top metal frame 584 has a numberof through-holes in alignment with tapped holes in the base metal ring582. Screws 580 can be extended through the holes in the top metal frame584, engaged with the tapped holes in the base metal ring 582, andtightened to sufficiently apply uniform pressure across the window 522.

Referring to FIG. 15, in a second approach, the window frame 584 a isprovided in a suitable alloy (other than Kovar®) or non-alloy material(e.g., ceramic) having a CTE closer to the window material. Severalmaterials developed for semiconductor applications are particularlysuitable, including silica fiber, and a copper/continuous-carbon-fiberalloy. See, e.g., Zweben, Carl, “Advanced Materials for OptoelectronicPackaging”, Electronic Packaging & Production Journal (Sep. 1, 2002).The window 522 is bonded to the frame 584 a. The frame 584 a is providedwith a number of holes therethrough allowing screws 580 therethrough tointerface with the rectangular base ring 582 bonded to the DMD base 550or the seal ring. In assembly, the window 522 is bonded to the frame 584a, and the base ring 582 is bonded to the DMD body 550 or seal ringusing a low vapor epoxy 554. Once the epoxy 554 has cured, the o-ring562 is positioned on the base ring 582, and the fused window/frameassembly is positioned onto the o-ring 562. Finally, the screws 580would be inserted through the holes in the frame 584a and tightened toapply uniform window pressure against the o-ring 562.

Referring to FIG. 16, according to a third approach, the window 522 isprovided with drilled holes and the window itself is screwed down to thebase ring 582. Holes can be drilled in fused silica using diamond drillbits or with lasers. In assembly, first, holes are drilled through thewindow material. Second, the base ring 582 is provided with tapped holesand bonded to the DMD body 550 or seal ring using a low vapor epoxy 554.Third, once the epoxy 554 has cured, the o-ring 562 is positioned on thebase ring 582. Fourth, the window 522 is properly positioned onto theo-ring 562. Finally, the screws 580 are inserted through the windowholes and tightened to apply uniform pressure against the o-ring 562.

In any of the embodiments described as using an epoxy bond, an adhesive,and preferably a polymer-based adhesive, can be used in place of theepoxy. The use of adhesives permits relatively easy removal of thebonded components, if necessary.

There are also additional methods and materials similar to theelastomeric seal that can be used to supply excellent hermetic seals,which can be better than the elastomeric seal. Referring to FIG. 17, onesuch method is the knife-edge seal, which generally uses a copper orlead gasket 600, typically of a grade for vacuum compatibility. In thismethod, a top component (such as a frame 602 with through holes 604) anda bottom component (such as a base ring 606 with threaded holes 608)both contain knife edges 610, 612 that dig into the gasket (on both itstop and bottom surfaces) as the top component is secured against thebottom component with screws (not shown). Small imperfections in theknife-edges 610, 612 are filled in with material as the gasket 600 isdeformed, and a very good hermetic seal is formed.

Referring to FIG. 18, another method is known generically as the C-seal,whereby a C-shaped material 650 is used to provide the seal between twoopposing components (such as a top frame 652 with through holes 654 anda bottom base ring 656 with threaded holes 658). In a classic “° C” sealthe open side 660 of the “C” 650 faces away from the sealed environment662. The seal compresses slightly when the joint is made up, e.g., thetop component is secured against the bottom component with screws (notshown). The elastic properties of the seal material maintain pressureagainst the surfaces of the sealing cavity. The seal material is softerthan the cavity surface. The softer seal material fills imperfections inthe cavity surface to create a leak-tight joint. There are furthervariations on this method, which supply even greater sealing capacity,such as an “energized” C-seal. Such seals, often termed Helioflex seals,are available from Garlock Helicoflex, Columbia, S.C.

Referring to yet another aspect of the invention, the mirrors on the DMDbase are preferably coated with a high reflectance (HR) coating adaptedto enhance UV-light reflection relative to the uncoated typicallyaluminum mirrors. The HR coating is preferably adapted for a 10° to 30°or 12° to 36° angle of incidence (depending upon mirror size). In the“ON” position, the mirror is tilted toward the incoming illumination by10° or 12°. Thus, the illumination will strike the mirrors at either 10°or 12°. Most maximum HR reflection coatings consist of dielectricmaterials which yield narrow bands of high reflectance at particularwavelengths. HR-dielectric layers work on the same principles asdielectric anti-reflection coatings. Quarter-wave thicknesses ofalternately high- and low-refractive index materials are applied to thealuminum mirror substrate to form a dielectric multilayer. By choosingmaterials of appropriate refractive indices, the various reflectedwavefronts can be made to interfere constructively in order to produce ahighly efficient reflector. The peak reflectance value is dependent uponthe ratio of refractive indices of the two materials, as well as thenumber of layer pairs. Increasing either increases the reflectance. Anoptimized coating at 10° or 12° angle of incidence reflection is 97% to99% at 193-nm (or a loss of only 1 to 3%) and 90% to 95% at 157-nm (or aloss of only 5 to 10%). As the angle of incidence increases from 10° or12°, the reflectance decreases. However, such losses are minimal for thefull range of movement of the mirrors. Additionally, a simple magnesiumfluoride coating deposited on the bare aluminum mirror will provide abroadband reflection in the 157-nm to 193-nm range of 86% to 88%, thusproviding more of a general-purpose approach, but with reducedreflection overall. This may be acceptable in some applications.

The HR coatings can be applied during manufacture of the DMD, or afterthe DMD has been manufactured, but before the window has been applied.To apply the HR coating during the manufacture of the DMD, a completedCMOS memory circuit (an SRAM cell) for the DMD superstructure isobtained, and an interlevel dielectric is provided over the metal-2layer of the CMOS. The dielectric is then planarized using a chemicalmechanical polish (CMP) technique which provides a completely flatsubstrate for DMD superstructure fabrication. Through the use of severalphotomask layers, the superstructure is formed with layers of aluminumand proprietary metal alloys for the address electrode (metal-3), hinge,yoke and mirror layers and hardened photo-resist for the sacrificiallayers (spacer-1 and spacer-2) that form the air gaps. The aluminum andmetal alloys are sputter-deposited and plasma-etched usingplasma-deposited SiO₂ as the etch mask. Later in the packaging flow, thesacrificial layers are plasma-etched to form the air gaps. The HRcoating is preferably deposited after the aluminum mirror layer isdeposited, and before the sacrificial layers are etched.

In the second method, the HR coating is applied after the DMD has beenmanufactured but before the UV-window is applied. Referring to FIG. 19,during the coating process, the micromirrors are preferably in their“flat” state, and the DMD unit is rotated to match the angle of theincoming laser beam (e.g., 20° for a 16-micron mirror package and 24°for a 13.7-micron mirror package). This allows coating of the underlyingstructure behind the mirrors, thus protecting these structures from theUV energy. The HR coating is applied preferably using a cold depositiontechnique, such as sputtering. The UV-window, in one of its manypossible embodiments, is then attached to the DMD unit.

Other Applications

There are other applications where the deep-UV DMD window assembly ofthe invention can be used. The DMD is, an ideal device for deliveringpatterns to semiconductor material, or other material, in the productionof integrated circuits (IC), or other optical lithography applications(such as polymer arrays). Currently, expensive, non-changeable,photomasks are used to provide the deprotection patterns for the etchingprocess. Photomasks, requiring sophisticated manufacturing techniquesand complex mathematical algorithms to design, are at the forefront ofthe microminiaturization of chips, enabling more functionality to beembedded within a smaller area. Photomasks are an integral component inthe lithographic process of semiconductor manufacturing. They consist ofhigh-purity quartz or glass plates containing precision patterns ofintegrated circuits, and are used as masters by chipmakers, and otherindustries, to optically transfer these images onto semiconductorwafers. Current advanced lithographic tools, such as deep-UV steppers,project light through a photomask and a high aperture lens. Thepredominant light wavelengths are 248-nm and 193-nm, although, asdiscussed below, 157-nm wavelengths are beginning to emerge. Theintensity of the light casts an image of the design for the device(i.e., the pattern on the photomask) onto a silicon wafer coated with alight sensitive material called photoresist. Using negative photoresistthe unexposed, or masked, portion of this material is then removed so itcan either be etched to form channels or be deposited with othermaterials. The process is reversed using positive photoresist. ICs aremanufactured layer by layer, so these selective deposition/removal stepsare repeated until the circuit is built. The current generation ofsemiconductors often has twenty-five or more layers, each requiring aunique photomask.

With the use of a DMD, the photomask is eliminated as the desiredpattern can be readily implemented on the DMD mirror array, provided theDMD has the necessary resolution. Using the appropriate light, the DMDmirrors can then cast, or direct, the image onto the silicon wafersubstrate, or other material, coated with the light-sensitivephotoresist. Since the host computer controls the DMD mirrors, thepattern can be changed rapidly by turning the appropriate mirrors ON orOFF. The masked portion of this material is then removed so it caneither be etched to form channels or be deposited with other materials.Presently, the DMD is not being used in these vacuum-UV and deep-UVapplications largely due to the current, commercially-available,UV-limited DMD window design. The UV-transmissable DMD window of thepresent invention in this patent can allow the use of the DMD in theseapplications.

In addition, optical lithography with 157-nm fluorine lasers is rapidlyemerging as a viable technology for the post-193-nm era. In fact, it maybecome the technology of choice for 100-nm to 70-nm nodes (i.e., smallphysical details). It is attractive for several reasons, the mostimportant being that it is fundamentally an extension of opticallithography at the longer wavelengths of 248 and 193 nm. Therefore, itholds the promise that the tool-manufacturing and wafer-processinginfrastructures can be adapted to it relatively easily, and thatoptical-resolution-enhancing techniques (phase-shifting masks, off-axisillumination, etc.) can be applied to it as well. However, this approachstill uses photomasks as described above for the 248-nm and 193-nmwavelengths. Thus, a window for the DMD allowing the transmission of157-nm would be advantageous as semiconductor processing moves in thisdirection.

The main difference between the main window embodiment and the deeper UVwavelength windows (such as down to 157-nm and even below) is the windowmaterial and coatings used for the shorter wavelengths. Currently, apreferred optical material for the window in vacuum-UV (VUV, generallydefined as 100-nm to 200-nm) applications is lens-quality calciumfluoride (CaF₂) (having a transmission of at least 50% for wavelengthsdown to 130-nm), particularly as disclosed in U.S. Pat. No. 6,242,136 toMoore et al., which is hereby incorporated by reference herein itsentirety. Other candidate materials include barium fluoride (BaF₂)(having a transmission of at least 50% for wavelengths down to 150-nm),strontium fluoride (SrF₂) (having a transmission of at least 50% forwavelengths down to 140-nm), lithium fluoride (LiF)(having atransmission of at least 70% for wavelengths down to 120-nm), magnesiumfluoride (MgF₂) (having a transmission of at least 65% for wavelengthsdown to 120-nm), and sodium fluoride (NaF)(having a transmission of atleast 50% for wavelengths down to 135-nm). The bonding of the window tothe frame is similar to that described in the above describedembodiments, although the bonding alloy may have different properties.

To optimize transmission for the 157-nm wavelength, fluorides are apreferred coating, as most oxide compounds (such as silicon dioxide orhafnium oxide) are too absorptive at 157-nm. For example, low index ofrefraction materials may include magnesium fluoride and aluminumfluoride, while high index materials may include lanthanum fluoride andgadolinium fluoride. Coating design and application techniques are verysimilar to those discussed above.

There have been described and illustrated herein embodiments of (i) aDMD, (ii) UV-transmissable window assemblies therefor, and (iii) methodsfor constructing the same. While particular embodiments of the inventionhave been described, it is not intended that the invention be limitedthereto, as it is intended that the invention be as broad in scope asthe art will allow and that the specification be read likewise. Thus, itis recognized that DMD windows for other UV wavelengths can beconstructed by the above method by using appropriate materials for thoseparticular wavelengths. Furthermore, while the invention has beendescribed particularly with respect to a DMD, it is recognized that theUV-transmissable window assemblies may have application in optical MEMSdevices, such as deformable and active mirror devices. It will also beappreciated by those skilled in the art that yet other modificationscould be made to the provided invention without deviating from itsspirit and scope as claimed.

1. A window assembly for a digital micromirror device (DMD) having anarray of individually addressable mirrors, comprising: a) a frameadapted in size to be coupled to the DMD and having an aperture at leastthe size of the mirror array, said frame about said aperture having afirst chamfered edge; b) a UV light transmissable window coupled withinsaid aperture, said window including a periphery provided with a secondchamfered edge, said first and second chamfered edges together defininga channel; and c) a brazing material provided in said channel thatprovides a seal between said window to said frame.
 2. A window assemblyaccording to claim 1, wherein: said seal is a hermetic seal.
 3. A windowassembly according to claim 1, wherein: said window is chosen to provideat least 50% transmission of light waves at and above 130-nm.
 4. Awindow assembly according to claim 1, wherein: said window is chosen toprovide at least 65% transmission of light waves at and above 120-nm. 5.A window assembly according to claim 1, wherein: said window is chosento provide at least 80% transmission of light waves at and above 185-nm.6. A window assembly according to claim 1, wherein: said window is afused silica glass window.
 7. A window assembly according to claim 1,wherein: said window is an argon fluoride (ArF) grade silica glasswindow.
 8. A window assembly according to claim 1, wherein: said windowconforms to Corning code
 7980. 9. A window assembly according to claim1, wherein: said window is made from a material selected from the groupcomprising calcium fluoride, barium fluoride, strontium fluoride,lithium fluoride, magnesium fluoride and sodium fluoride.
 10. A windowassembly according to claim 1, further comprising: a dielectric coatingon said window.
 11. A window assembly according to claim 1, wherein:said frame comprises a metal alloy.
 12. A window assembly according toclaim 11, wherein: said metal alloy is a nickel-cobalt-iron alloy.
 13. Awindow assembly according to claim 12, wherein: said metal alloy isKovar®.
 14. A window assembly according to claim 12, wherein: said metalalloy is comprised of approximately 29% nickel, 17% cobalt, 0.30%manganese, 0.20% silicon, 0.02% carbon, and a balance of iron.
 15. Awindow assembly according to claim 1, wherein: said brazing material isa lead/silver alloy brazing material.
 16. A window assembly according toclaim 15, wherein: said lead/silver alloy comprises approximately 97.5%lead and 2.5% silver.
 17. A window assembly according to claim 1,wherein: said window includes an inside and outside, and at least one ofsaid inside and said outside is provided with one or more opticalcoatings.
 18. A window assembly according to claim 17, wherein: at leastone of said optical coatings is a narrowband antireflection (AR)coating.
 19. A window assembly according to claim 17, wherein: at leastone said optical coatings is a multilayer stack coating.
 20. A windowassembly according to claim 19, wherein: said stack coating is optimizedfor an angle of incidence between 0° and 12°.
 21. A window assemblyaccording to claim 17, wherein: at least one of said optical coatings isoptimized for a dual angle-of-incidence.
 22. A digital micromirrordevice (DMD), comprising: a) a base element provided with a twodimensional array of individually addressable mirrors; b) a framecoupled to the base element and having an aperture at least the size ofthe mirror array; and c) a window coupled within said aperture andbonded to said frame, said window comprising a material adapted topermit high transmission of UV wavelength light therethrough.
 23. A DMDaccording to claim 22, wherein: said frame has a first chamfered edgeabout said aperture, said window has a periphery with a second chamferededge, and said first and second chamfered edges define a channel, andwherein said window is bonded to said frame with a brazing materialprovided in said channel.
 24. A DMD according to claim 22, wherein: abond between said window and said frame forms a hermetic seal.
 25. A DMDaccording to claim 22, wherein: said window is a fused silica glasswindow.
 26. A DMD according to claim 22, wherein: said window is anargon fluoride (ArF) grade silica glass.
 27. A DMD according to claim22, wherein: said window material is selected from the group comprisingcalcium fluoride, barium fluoride, strontium fluoride, lithium fluoride,magnesium fluoride and sodium fluoride.
 28. A DMD according to claim 27,further comprising: a dielectric coating on said window.
 29. A DMDaccording to claim 22, wherein: said frame comprises anickel-cobalt-iron alloy.
 30. A DMD according to claim 22, wherein: saidwindow includes an inside and outside, and at least one of said insideand said outside is provided with one or more optical coatings.
 31. ADMD according to claim 30, wherein: at least one of said opticalcoatings is a narrowband antireflection (AR) coating.
 32. A DMDaccording to claim 30, wherein: at least one of said optical coatings isa multilayer stack coating.
 33. A DMD according to claim 32, wherein:said stack coating is optimized for an angle of incidence between 0° and12°.
 34. A DMD according to claim 22, wherein: said mirrors are providedwith a coating to increase reflectance.
 35. A DMD according to claim 34,wherein: said coating is optimized for an angle of incidence between 10°to 36° angle of incidence.
 36. A DMD according to claim 34, wherein:said mirrors are manufactured from aluminum, and said coating is adielectric material.
 37. A digital micromirror device (DMD), comprising:a) a base element provided with a two dimensional array of individuallyaddressable mirrors; b) a frame coupled to the base element and havingan aperture at least the size of the mirror array, said frame comprisedof a material having a first coefficient of thermal expansion; c) awindow coupled within said aperture of said frame, said windowcomprising a material adapted to permit high transmission of UVwavelength light therethrough and having a second coefficient of thermalexpansion significantly different than said first coefficient of thermalexpansion; and d) a seal between said window and said frame.
 38. A DMDaccording to claim 37, wherein: said first and second coefficients ofthermal expansions are different by approximately an order of magnitude.39. A DMD according to claim 37, wherein: said seal is a hermetic seal.40. A digital micromirror device (DMD), comprising: a) a base elementhaving a first side and a second side, said first side provided with atwo dimensional array of individually addressable mirrors; b) a Kovar®alloy frame having an aperture at least the size of the mirror array; c)an argon fluoride grade fused silica window between said frame and saidbase element; and d) a hermetic seal between said window and said baseelement.
 41. A digital micromirror device (DMD), comprising: a) a baseelement having a first side and a second side, said first side providedwith a two dimensional array of individually addressable mirrors; b) aframe having an aperture at least the size of the mirror array; and c) awindow between said frame and said base element, said window comprisinga material adapted to permit at least 90% transmissibility of UVwavelengths at 185-nm and above, wherein said window and said baseelement are coupled in a manner which defines a hermetic seal about saidarray of mirrors.
 42. A method of assembling a window unit for a digitalmicromirror device (DMD), comprising: a) providing a frame having anaperture; b) providing a fused silica glass window; c) bonding saidwindow in said aperture of said frame to form a window unit; and d)optimizing said window for transmission of light at one or moreparticular wavelengths.
 43. A method according to claim 42, wherein:said window and said frame define a channel at the junctiontherebetween, and said bonding includes providing a brazing alloy insaid channel and heating said window, said frame and said brazing alloyto effect a hermetic seal between said window and said frame.
 44. Amethod according to claim 42, wherein: said window includes an insideand an outside, and said optimizing includes applying to at least one ofsaid inside and said outside of said window at least one coating adaptedto increase UV light transmission through said window.
 45. A methodaccording to claim 44, wherein: prior to applying said coating, maskingportions of said window unit.
 46. A method according to claim 44,wherein: said applying utilizes a low temperature process.
 47. A methodaccording to claim 44, wherein: said applying comprises sputtering. 48.A method according to claim 42, wherein: said optimizing includesoptimizing said window for a particular angle of incidence.
 49. A methodaccording to claim 42, wherein: said optimizing includes optimizing saidwindow for a range of angles-of-incidence.
 50. A method according toclaim 42, further comprising: e) installing said window unit to a baseelement of a DMD, said DMD having a plurality of individuallyaddressable mirrors.
 51. A method according to claim 50, furthercomprising: f) coating said mirrors with a relatively higher reflectancecoating.